Aluminum alloy parts intended to be used in the aeronautical sector, or in other sectors in which they are likely to be exposed to corrosion risks that may prove problematic, generally receive, prior to application, a surface treatment with the aim of protecting them against corrosion.
One of the techniques most widely used for this purpose is anodizing, also called anodic oxidation, which consists of forming, on the surface of the part, a porous layer of aluminum oxides/hydroxides, called an anodic layer, by applying a current to the part immersed in an electrolytic bath containing an electrolyte of the strong acid type, the part constituting the anode of the electrolytic device. The anodic layer thus formed on the surface of the part, after undergoing a sealing post-treatment, protects the part against corrosion. This anodic layer also constitutes a substrate suitable for conventional paint systems.
The electrolytic baths employed at present for anodizing aluminum alloy parts, which provide the most advantageous performance notably in terms of protecting the part against corrosion, mechanical bonding of paint coatings to the surface of the part, and reduction in fatigue strength, are formed on the basis of hexavalent chromium. However, chemicals containing hexavalent chromium have proved to be harmful to health and to the environment.
In order to avoid the use of substances based on hexavalent chromium for anodizing aluminum alloy parts, anodizing processes have been proposed in the prior art using other strong acids in the electrolytic bath, and in particular sulfuric acid. However, none of these baths gives satisfactory performance in terms simultaneously of protection of the part against corrosion, adherence of conventional paint systems on the part, and reduction in fatigue strength of the part. The performance proves insufficient notably in respect of the requirements imposed in the aeronautical sector.